Process for the preparation of alkoxyalkanoic acids

ABSTRACT

A process for the preparation of an alkoxyalkanoic acid by reacting the corresponding alkoxyalkanol with a stable free radical nitroxide in the presence of a NO x  -generating compound, a solvent and an oxidant, at a temperature in the range of from about 0° C. to about 100° C. and thereafter separating out the alkoxyalkanoic acid.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of alkoxyalkanoic acids by the oxidation of the corresponding alkoxyalkanols in the presence of a stable free radical nitroxide, a NO_(x) -generating compound, a solvent and an oxidant.

BACKGROUND OF THE INVENTION

Alkoxyalkanoic acids are useful as anionic detergents or emulsifying agents. These acids, being composed of only the elements C, H and O, do not pose the environmental problems that other detergents containing heteroatoms such as N, S and P pose. Commercially, the alkoxyalkanoic acids are prepared in a two-step process of first reacting an alkoxyalkanol with sodium and then reacting the resultant alkoxide with the sodium salt of chloroacetic acid.

It is also known to convert alkoxyalkanols such as methyl carbitol to the corresponding carboxylic acids by oxidizing them with nitric acid. However, not all of the nitric acid can be separated by distillation, and the reaction product contains nitric acid, which is corrosive and therefore undesirable. In addition, cleavage of the ether linkages occurs to a large degree during this process.

Japanese Patent No. 50-96519 issued on Jul. 31, 1975, discloses a process for the preparation of carboxylic acid salts by the liquid phase dehydrogenation of alcohols with caustic alkali in the presence of precious metal catalysts, including palladium. This process uses a relatively high temperature, 100° C.-270° C. These high temperatures can degrade the ether linkages, especially in the highly ethoxylated alcohols.

It is known to use nitroxyl radicals/oxoammonium salts in the oxidation of primary alcohols to produce aldehydes and acids and secondary alcohols to ketones (Journal of Organic Chemistry, Vol. 52 (12), pp. 2559-2562, Pure and Applied Chemistry, Vol. 62(2), 1990, pp. 217-222, and Journal of Organic Chemistry, Vol. 55, 1990, pp. 462-466). The primary products produced in these processes are aldehydes and the stoichiometrically consumed oxidant is hypochlorite.

It is generally more difficult to oxidize alkoxyalkanols than alkanols as it is difficult to oxidize alkoxyalkanols without splitting the molecular chain at the ether linkage and thereby produce a large proportion of undesired by-product.

It has been found that alkoxyalkanoic acids can be produced in high yields and with high selectivities from alkoxyalkanols without producing large amounts of highly corrosive, difficult to separate, by-products. This can be accomplished by using catalytic amounts of a stable free radical nitroxide, a NO_(x) -generating compound, a solvent, and an oxidant.

SUMMARY OF THE INVENTION

This invention relates to a process for the preparation of an alkoxyalkanoic acid of the formula RO(CH₂ CHR'O)_(n) CH₂ CO₂ H wherein R is an alkyl group of from about 1 to about 22 carbon atoms, R' is hydrogen or methyl or mixtures thereof (on the individual molecule) and n is an integer of from about 1 to about 500 which comprises reacting the corresponding alkoxyalkanol with a stable free radical nitroxide having the formula: ##STR1## wherein (1) (a) each of R₁, R₂, R₃ and R₄ is an alkyl, aryl or substituted alkyl group having 1 to about 15 carbon atoms, and (b) R₅ and R₆ (i) each is an alkyl group having 1 to about 15 carbon atoms provided that R₁ -R₆, are not all alkyl groups, or a substituted alkyl group having 1 to about 15 carbon atoms wherein the substituent is hydrogen, cyano, --CONH₂, --OCOCH, OCOC₂ H₅, carbonyl, alkenyl wherein the double bond is not conjugated with the nitroxide moiety, or --COOR wherein R of the --COOR group is alkyl or aryl, or (ii) together form part of a ring that contains at least 3 carbon atoms and up to two heteroatoms of O or N, or (2) the ##STR2## moiety and the ##STR3## moiety individually are aryl, or (3) the ##STR4## moiety and the ##STR5## moiety together form a bicyclic ring with the proviso that the group directly adjacent to the N-O moiety is a bridgehead C-H, or a fully alkylated carbon, in the presence of a NO_(x) -generating compound, a solvent and an oxidant, at a temperature in the range of from about 0° C. to about 100° C. and thereafter separating out the alkoxyalkanoic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process converts alkoxyalkanols of the formula RO(CH₂ CHR'O)_(n) CH₂ CH₂ OH (I) wherein R is an alkyl group having from about 1 to about 22 carbon atoms, preferably from about 11 to about 18 carbon atoms,R' is hydrogen or methyl or mixtures thereof (on the individual molecule) and n represents the average number of oxyalkylene groups and is an integer of from about 1 to about 500, preferably from about 1 to about 50, more preferably from about 1 to about 12, and most preferably from about 2 to about 9, to the corresponding alkoxyalkanoic acids of the formula RO(CH₂ CHR'O)_(n) CH₂ CO₂ H (II) by contacting the alkoxyalkanol with a stable free radical nitroxide in the presence of a NO_(x) -generating compound, a solvent and an oxidant at a temperature in the range of from about 0° C. to about 100° C. and thereafter separating out the alkoxyalkanoic acid. The R group in the above formula I can be substituted with any substituent which does not interfere with the oxidation of the hydroxy group. Such substituents include --OR", --CH₃, --COOH, CONH₂ and COOR" wherein R' is an alkyl or aryl group. The process of the instant invention is particularly suited to detergent range ethoxylated, or propoxylated alcohols with alkyl chains (R) of about 8 to about 20, preferably about 11 to about 18 carbon atoms. The R' groups on an individual molecule can be hydrogen, methyl, or mixtures thereof. For example, straight ethoxylated, straight propoxylated and mixed ethoxylated-propoxylated detergent alcohols are commercially available. The number of such alkoxylate groups, CH₂ CHR'O, ranges from about 1 to about 20. Commercially, detergent range ethoxylate alcohols are available with an average of 3, 7, 9 and 12 ethoxylate units per molecule. Others can be readily prepared. In a preferred embodiment, the alkoxyalkanol reactant is an ethoxylated alcohol which has had the unreacted alcohols and lower ethoxylates topped off in order to give an ethoxylate having about four ethylene oxide units per molecule.

The term "stable free radical nitroxide" as used herein shall mean a free radical nitroxide that can be prepared by conventional chemical methods and will exist long enough to be used in a subsequent chemical reaction or examined in a static system by normal methods of spectroscopy. Generally, the stable free radical nitroxides of the present invention have a half life of at least one year. The term "stable free radical" shall also be understood to include the precursor to a stable free radical from which the stable free radical may be produced in situ.

The stable free radical nitroxides, as used in the present process, are precursors to catalysts, i.e., oxoammonium salts, active for the oxidation of alkoxyalkanols to the corresponding acids. These catalysts are generated in situ by the oxidation of a stable free radical nitroxide to an oxoammonium salt with an oxygen-containing oxidant. The stable free radical nitroxide can be obtained by the oxidation of amines or hydroxylamines.

The stable free radical nitroxides which are suitable for use in the instant invention have the formula: ##STR6## wherein each of R₁, R₂, R₃ and R₄ is an alkyl, aryl or substituted alkyl groups and no hydrogen is bound to the remaining valences on the carbon atoms bound to the nitrogen. As used herein, the term "alkyl" is meant to include cycloalkyl. The alkyl (or substituted) groups R₁ -R₄ may be the same or different, and preferably contain 1 to 15 carbon atoms. Preferably, R₁ -R₄ are methyl, ethyl, or propyl groups. In addition to hydrogen, the substituents may include, halogen, oxygen, nitrogen and the like.

The remaining valences (R₅ and R₆) in formula III above may be satisfied by any atom or group except hydrogen which can bond covalently to carbon, although some groups may reduce the stabilizing power of the nitroxide and are undesirable. When R₁, R₂, R₃ and R₄ are each alkyl groups, however, at least one of R₅ and R₆ must be an aryl group. Preferably, R₅ and R₆ are substituted alkyl groups having 1 to about 15 carbon atoms wherein the substituent is selected from halogen, cyano, --COOR, wherein R is alkyl or aryl, --CONH₂, --OCOC₂ H₅, carbonyl, or alkenyl where the double bond is not conjugated with the nitroxide moiety, or alkyl groups of 1 to about 15 carbon atoms.

The remaining valences (R₅ and R₆ ) in formula III above may also form a ring containing at least three carbon atoms and up to two heteroatoms, such as O or N. R₅ and R₆ can, for example, form a five-membered ring containing 3 carbon atoms and up to two heteroatoms, such as O or N, a five-membered ring containing 4 carbon atoms, a six-membered ring containing 5 carbon atoms, a seven-membered ring containing 6 carbon atoms, an eight-membered ring containing 7 carbon atoms, etc. For purposes of this invention, it is preferred that R₅ and R₆ together form a five-membered ring, a six-membered ring, a seven-membered ring, or an eight-membered ring, although larger rings would also be suitable. Examples of suitable compounds having the structure above and in which R₅ and R₆ form part of the ring are 2,2,6,6,-tetramethylpiperidine-1-oxyl, 2,2,5,5-tetramethyl pyrrolidin-1-oxyl, 2,2,7,7-tetramethylcycloheptan-1-oxyl, mixtures thereof, and the like. It is understood that these compounds may contain substituents which do not interfere with the reaction.

The ##STR7## and the ##STR8## moieties in formula III above can individually be aryl, i.e., ##STR9## Examples of suitable compounds having the structure above in which the ##STR10## and the ##STR11## moieties are individually aryl are diphenylamine-N-oxyl, phenyl tertiary-butylamine-N-oxyl, 3-methylphenyl phenylamine-N-oxyl, 2-chlorophenyl phenylamine-N-oxyl and the like. These compounds may be substituted with an substituents which do not interfere with the reaction.

The ##STR12## and the ##STR13## moieties in formula III above can also form a bicyclic ring wherein the group adjacent to the N-O moiety is either a bridgehead C-H or a quaternary carbon. As used herein, the term "bridgehead C-H" refers to a tertiary carbon which is common to both rings of the bicyclic ring system. As used herein, "a quaternary carbon" refers to a fully substituted carbon atom having alkyl, aryl or substituted alkyl groups having 1 to about 18 carbon atoms as substituents. Examples of suitable compounds having the structure above in which the ##STR14## and the ##STR15## moieties form a bicyclic ring are 2-azabicyclo-[2.2.1]heptan-2-oxyl, 2-azabicyclo[2.2.2]-3,3-dimethyloctan-2-oxyl, 3-azabicyclo[3.2.2]-2,2, 4,4-tetramethylnonan-3-oxyl and the like. These compounds may be substituted with any substituents which do not interfere with the reaction.

In a preferred embodiment, the stable free radical nitroxide is a piperidine-1-oxyl having the formula: ##STR16## wherein each of R₇, R₈, R₉ and R₁₀ is an alkyl, aryl or substituted alkyl group having 1 to about 15 carbon atoms and no hydrogen is bound to the remaining valences on the carbon atoms bound to the nitrogen, and each of R₁₁ and R₁₂ is alkyl, hydrogen, aryl or a substituted heteroatom. As used herein, the term "alkyl" is meant to include cycloalkyl. The alkyl (or substituted) groups R₇ -R₁₀ may be the same or different, and preferably contain 1 to 15 carbon atoms. Preferably, R₇ -R₁₀ are methyl, ethyl, or propyl groups. In addition to hydrogen, the substituents may include, halogen, oxygen, nitrogen and the like. Typically, one of R₁₁ and R₁₂ is hydrogen, with the other one being a substituted heteroatom which does not interfere with the reaction. Suitable substituted heteroatoms include --OR, ##STR17## --⁺ NMe₃ Cl--, --O--SO₃ H, --O--polymer and the like.

In a particularly preferred embodiment, the nitroxide is selected from the group consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-hydroxy-2,2, 6,6-tetramethylpiperidine-1-oxyl, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-sulfate, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-ethoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-carbamoyl-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-benzoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-pivaloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-dodecyloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-dodecanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-octanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and mixtures thereof, with 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetylamino-2,2, 6,6-tetramethylpiperidine-1-oxyl, and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl being especially preferred.

The NO_(x) -generating compound in the present process is typically nitric acid. However, any compound which serves to generate NO_(x) during the course of the reaction, such as, for example, an alkali metal nitrosodisulfonate, and which does not interfere with the reaction would be suitable. While not wishing to be bound by any particular theory, it is believed that nitrogen oxides (NO_(x)) are generated in the reaction and are the active species in the reaction.

As used herein, the term "nitric acid" refers to nitric acid, fuming nitric acid or nitrous acid generated by contacting a nitrate or nitrite salt such as, for example, an alkali metal salt, a tetraalkylammonium salt, an alkaline earth salt or a rare earth salt, with a strong acid such as, for example, a mineral acid. The nitric acid suitable for use as a NO_(x) -generating compound in the present invention typically has a concentration in the range of from about 50 percent to about 100 percent, preferably about 70 percent. Generally, an amount of nitric acid in the range of from about 5 mole percent to about 200 mole percent, basis the moles of starting alkoxyalkanol is utilized. The nitric acid is typically added to the reaction mixture after all of the other reactants have been added.

The reaction of the present invention is carried out in the presence of a solvent. The solvent is generally one in which the alkoxyalkanol is readily soluble. Solvents which are most suitable are those which are inert in the reaction. The solvent may be added to the reaction mixture, or alternatively, the nitroxide may be dissolved in the solvent prior to addition of the nitroxide to the reaction medium. The solvent is typically selected from the group consisting of alkyl ethoxycarboxylates, alkyl carboxylates, chlorinated hydrocarbons, chorofluorohydrocarbons, hydrocarbons, polyethers, dipolar aprotic solvents and mixtures thereof. Tertiary alcohols may also be utilized as solvents in the process of the present invention, however, reaction temperatures in the range of about 40° C. to about 60° C. are required in order for the reaction to proceed. In a preferred embodiment, the solvent is selected from the group consisting of alkyl ethoxycarboxylate, dichloromethane, carbon tetrachloride, proprionic acid, heptane, diglyme, tertiary amyl alcohol, freon and mixtures thereof. The amount of solvent utilized in the process is typically in the range of from about 0.5:1 to about 100:1, preferably from about 1:1 to about 10:1, basis the weight of the starting alkoxyalkanol.

The process of the present invention is also carried out in the presence of an oxidant. The oxidants suitable for use in the instant invention are those compounds which are capable, in the presence of a NO_(x) -generating compound, of oxidizing the stable free radical nitroxide to the oxoammonium salt. Suitable oxidants include oxygen-containing gases such as pure oxygen and oxygen in air. Whereas pure oxygen can is preferred to accomplish the desired conversion, the oxygen can also be diluted with an inert gas such as nitrogen, helium, argon, or other similar gas. For purposes of increasing the reaction rate, higher O₂ pressures such as, for example, up to about 2000 psig can be utilized. In a preferred embodiment, pure oxygen is used as the oxidant and it is bubbled into the reaction solution. In another embodiment, the reaction solution can be exposed to compressed air under pressure.

The amounts and concentrations of the reactants utilized in the process of the instant invention can vary within wide ranges. The amount of stable free radical nitroxide is typically in the range of from about 0.01 mole percent to about 200 mole percent, preferably from about 5 mole percent to about 20 mole percent, basis the number of moles starting alkoxyalkanol. Generally, the amount of NO_(x) -generating compound used is in the range of from about 5 mole percent to about 200 mole percent, basis the number of moles of alkoxyalkanol.

The process of the present invention is typically conducted under mild conditions, with good results being obtained using a temperature in the range of from about 0° C. to about 100° C., preferably about 20° C. to about 70° C., and most preferably, about 35° C. to about 65° C. Higher pressures can result in increased reaction rates. Pressures in the range of from about atmospheric pressure up to about 2000 psig can be employed with good results.

The process of the instant invention can be carried out either batchwise or continuously, using a stirrer equipped reactor or other well known contacting technique to achieve adequate mixing. Preferred reaction conditions, e.g., temperature, pressure, flow rates, etc., vary somewhat depending on the specific nitroxide utilized and on the concentration of the nitroxide.

The process of the instant invention can be carried out in a variety of ways. For example, 0.03 moles of alkoxyalkanol, and 0.003 moles of the nitroxide, and solvent may be added to the reaction vessel, followed by the addition of 0.011 moles of 70 percent nitric acid and bubbling oxygen through the reaction mixture. Following the reaction, the product may be separated from the reaction mixture using conventional procedures such as extraction using a suitable extraction solvent such as, for example, ethyl acetate; evaporation wherein the solvent is stripped from the reaction mixture by using heat or vacuum. Phase separation of the final product mixture can take place at 100° C. with water. The reaction product can be purified by a number of conventional means such as high temperature water washing or distillation.

Depending upon process conditions and the nitroxide used, the yields of alkoxyalkanoic acid obtained by this invention can be very high, with greater than about 98% of starting material being converted. The products produced by the instant process can be used in a variety of detergent applications or emulsifying agents. For example, these products can be used in light duty dishwashing liquids, shampoos, heavy duty laundry liquids and heavy duty powders.

The ranges and limitations provided in the instant specification and claims are those which are believed to particularly point out and distinctly claim the present invention. It is, however, understood that other ranges and limitations which perform substantially the same function in the same or substantially the same manner to obtain the same or substantially the same result are intended to be within the scope of the instant invention as defined by the instant specification and claims.

The process of this invention will be further described by the following embodiments which are provided for illustration and are not to be construed as limiting the invention.

ILLUSTRATIVE EMBODIMENTS

In the following examples, the starting alkoxyalkanol was a NEODOL®Ethoxylate 23-3T alcohol which was prepared by ethoxylating a mixture of C₁₂ and C₁₃ substantially straight chain alcohols (C₁₂ :C₁₃ ˜40:60) to form an ethoxylate alcohol having about 3 ethylene oxide units per molecule and then topping off the unreacted alcohols and lower ethoxylates so that the final product has less than about 5 percent by weight of the starting alkanol.

EXAMPLE 1

12 Grams of NEODOL® Ethoxylate 23-3T, 0.5 grams of 2,2,6,6-tetramethylpiperidine-1-oxyl, 1 gram of 70 percent nitric acid and 25 milliliters of triglyme were charged to a 100 milliliter round bottomed flask. O₂ was bubbled through this mixture at a rate of 35 milliliters/minute at ambient pressure. The reaction temperature was held at 35° C. over a 4-hour period. The results are presented in Table I.

EXAMPLE 2

12 Grams of NEODOL® Ethoxylate 23-3T, 0.5 grams of 2,2,6,6-tetramethylpiperidine-1-oxyl, 1 gram of 70 percent nitric acid and 25 milliliters of an alkyl ethoxycarboxylate (a mixture of C₁₂ and C₁₃ alcohols having about 4 ethylene oxide units per molecule) were charged to a 100 milliliter round bottomed flask. O₂ was bubbled through this mixture at a rate of 35 milliliters/minute at ambient pressure. The reaction was held at 45° C. over a 5-hour period. The results are presented in Table I.

EXAMPLE 3

12 Grams of NEODOL® Ethoxylate 23-3T, 0.5 gram of 2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acid and 25 milliliters of dichloromethane were charged to a 100 milliliter round bottomed flask. The reaction mixture was sparged with air at a rate of 0.7 liters/minute at ambient and held at 20° C. over a 6-hour period. The results are presented in Table I.

EXAMPLE 4

12 Grams of NEODOL® Ethoxylate 23-3T, 1 gram of 2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acid, and 25 milliliters of n-heptane were charged to a 100 milliliter round bottomed flask. O₂ was bubbled through this mixture at a rate of 35 milliliters/minute at ambient pressure. The reaction temperature was held at 35° C. over a 3-hour period. The results are presented in Table I.

COMPARATIVE EXAMPLE A

Comparative Example A was carried out in a manner similar to Example 1 except that no nitroxide was used. The results are presented in Table I.

COMPARATIVE EXAMPLE B

Comparative Example B was carried out in a manner similar to Example 1 except that no nitric acid was used. The results are presented in Table I.

As can be seen in Table I, nitroxide and nitric acid are necessary for the oxidation of the alkoxyalkanol to proceed.

                  TABLE I                                                          ______________________________________                                         Oxidation of Alkoxyalkanols to Alkoxyalkanoic acids                                                % Selectivity                                                        % Conversion                                                                             Alkoxyalkanoic Acids                                       ______________________________________                                         Example 1   >99         >99                                                    Example 2   95          93                                                     Example 3   >99         95                                                     Example 4   97          85                                                     Comparative <2          0                                                      Example A                                                                      Comparative 0           0                                                      Example B                                                                      ______________________________________                                     

What is claimed is:
 1. A process for the preparation of an alkoxyalkanoic acid of the formula RO(CH₂ CHR'O)_(n) CH₂ CO₂ H wherein R is an alkyl group of from about 1 to about 22 carbon atoms, R' is hydrogen or methyl or mixtures thereof (on the individual molecule) and n is an integer of from about 1 to about 500, which comprises reacting the corresponding alkoxyalkanol with a stable free radical nitroxide having the formula: ##STR18## wherein (1) (a) each of R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substituted alkyl group having 1 to about 15 carbon atoms, and (b) R₅ and R₆ (i) each is an alkyl group having 1 to about 15 carbon atoms provided that R₁ -R₆ are not all alkyl groups, or a substituted alkyl group having 1 to about 15 carbon atoms wherein the substituent is hydrogen, cyano, --CONH₂, --OCOCH, OCOC₂ H₅, carbonyl, alkenyl wherein the double bond is not conjugated with the nitroxide moiety, or --COOR wherein R of the --COOR group is alkyl or aryl, or (ii) together form part of a ring that contains at least 3 carbon atoms and up to two heteroatoms of O or N, (2) the ##STR19## moiety and the ##STR20## moiety individually are aryl, or (3) the ##STR21## moiety and the ##STR22## moiety together form a bicyclic ring with the proviso that the group directly adjacent to the N-O moiety is a bridgehead C-H, or a fully alkylated carbon, in the presence of a NO_(x) -generating compound, a solvent and an oxidant, at a temperature in the range of from about 0° C. to about 100° C. and thereafter separating out the alkoxyalkanoic acid.
 2. The process of claim 1 wherein said alkoxyalkanol has a carbon number in the range from about 11 to about
 18. 3. The process of claim 1 wherein n is an integer in the range of from about 1 to about
 50. 4. The process of claim 1 wherein the stable free radical nitroxide is selected from the group consisting of 2,2,6,6,-tetramethylpiperidine-1-oxyl, 2,2,5,5-tetramethyl pyrrolidin-1-oxyl, 2,2,7,7-tetramethylcycloheptan-1-oxyl, and mixtures thereof.
 5. The process of claim 1 wherein the stable free radical nitroxide has the formula: ##STR23## wherein each of R₇, R₈, R₉ and R₁₀ is an alkyl, aryl or heteroatom substituted alkyl group having 1 to about 15 carbon atoms and each of R₁₁ and R₁₂ is alkyl, hydrogen, aryl or a substituted heteroatom.
 6. The process of claim 5 wherein the stable free radical nitroxide is selected from the group consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-ethoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-carbamoyl-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-benzoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-pivaloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-dodecyloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-dodecanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-octanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and mixtures thereof.
 7. The process of claim 6 wherein the stable free radical nitroxide is selected from the group consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl and mixtures thereof.
 8. The process of claim 1 wherein the solvent is selected from the group consisting of alkyl ethoxycarboxylates, alkyl carboxylates, chlorinated hydrocarbons, chorofluorohydrocarbons, hydrocarbons, polyethers, dipolar aprotic solvents, tertiary alcohols and mixtures thereof.
 9. The process of claim 1 wherein the solvent is selected from the group consisting of alkyl ethoxycarboxyl ate, dichloromethane, carbon tetrachloride, proprionic acid, heptane, diglyme, tertiary amyl alcohol, freon and mixtures thereof.
 10. The process of claim 1 wherein said NO_(x) -generating compound is nitric acid.
 11. The process of claim 10 wherein said nitric acid is selected from the group consisting of fuming nitric acid, nitrous acid generated by contacting an alkali metal nitrite with mineral acid, nitric acid generated by contacting an alkali metal nitrate with mineral acid, and mixtures thereof.
 12. The process of claim 10 wherein the amount of nitric acid is in the range of from about 5 mole percent to about 200 mole percent, basis the moles of starting alkoxyalkanol.
 13. The process of claim 1 wherein said alkoxyalkanol is contacted with said stable free radical nitroxide and said solvent, followed by the addition thereto of said NO_(x) -generating compound.
 14. The process of claim 13 wherein the amount of stable free radical nitroxide is in the range of from about 0.01 mole percent to about 200 mole percent, basis the number of moles of alkoxyalkanol.
 15. The process of claim 13 wherein the amount of NO_(x) -generating compound is in the range of from about 5 mole percent to about 200 mole percent, basis the number of moles of alkoxyalkanol.
 16. The process of claim 1 wherein said process is carried out in the presence of an oxidant.
 17. The process of claim 16 wherein said oxidant is an oxygen-containing gas.
 18. The process of claim 17 wherein said oxygen-containing gas is selected from the group consisting of pure oxygen and air.
 19. The process of claim 1 wherein said process is carried out at a temperature in the range of from about 20° C. to about 70° C. and a pressure in the range of atmospheric pressure to about 2000 psi. 